Due to production variances, the exact low-frequency corner of a microphone sensor is subject to variance. As a consequence, the phase response at low frequency is also subject to variance. E.g. if the LF-corner varies between 40 and 60 Hz, the phase response at 200 Hz varies between 11.3 and 16.7 deg. For acceptable beamforming however, the phase difference between the microphones of a matched pair with 12 mm port distance should be smaller than 2 degrees at 200 Hz; for accurate beamforming the phase difference should be smaller than 0.5 degrees. In some cases with smaller port spacing (e.g. on a faceplate) the requirement is 0.36 degrees at 170 Hz.
Currently, the phase difference between microphones is guaranteed by selection (to create matched pairs) or by sorting (to create arrays of microphones). The production and further assembly of these microphones requires careful handling in order not to alter the correct sequence.
Different solutions in this area may be seen in U.S. Pat. No. 6,914,992, US2004/179703, US2015/0137834, US2004/0179703, U.S. Pat. No. 9,148,729, US2015/0245143, US2014/0086433, US2016/0337753, US2014/0264652 and U.S. Pat. No. 8,170,237.
A first aspect, the invention relates to a phase correcting system comprising:                a first input terminal and a second input terminal both being configured to be connected to terminals of a transducer,        a first transport element configured to receive a signal from the first input terminal and feed a corresponding signal to an output terminal, and        a feedback element having:        a feedback entry conductor connected to the first transport element,        a feedback exit conductor connected to the second input terminal and        a circuit configured to receive a first signal from the feedback entry conductor and output, on the feedback exit conductor, a second signal as a low pass filtered first signal, the circuit having a variable cut-off frequency of the low pass filtering.        